1. Technical Field
The present invention relates to a photoelectric sensor used to detect an object, particularly to a photoelectric sensor suitable for use of detection of a thin object.
2. Related Art
In a photoelectric sensor that is used to detect a thickness portion of a thin object in a substrate conveying line or the like, since a complete light interception state is hardly generated depending on a detection target object, a detection signal is turned on based on a condition that an amount of received light is lower than a predetermined threshold.
In a conventional example concerning a disposition of this kind of sensor, as shown in FIG. 10, a phototransmitter 501 and an optical receiver 502 of a transmission type photoelectric sensor are disposed such that a detection target region is sandwiched therebetween, and a decrease of the amount of received light is detected when light traveling from the phototransmitter 501 toward the optical receiver 502 is partially intercepted by a substrate 503 (for example, see Japanese Unexamined Patent Publication No. 2009-216489). As shown in FIG. 11, the phototransmitter 501 and the optical receiver 502 are disposed while vertically deviated from each other, and the phototransmitter outputs the light such that the light obliquely traverses the detection target region (for example, see Japanese Unexamined Patent Publication No. 2007-258386).
In Japanese Unexamined Patent Publication Nos. 2009-216489 and 2007-258386, the substrate conveyed along a horizontal direction is detected in a predetermined position on a conveying path. Occasionally, the photoelectric sensor is used to detect vertical movement of the thin object. For example, in Japanese Unexamined Patent Publication No. 6-69323, a carrier in which a plurality of wafers are accommodated while arrayed in a vertical direction is vertically moved, and the wafers are sequentially put in an optical path of the transmission type photoelectric sensor to perform the detection, or a polarizing filter is attached to the optical receiver in order to prevent the incidence of the light reflected from the wafer on a stage higher than the detection target to the optical receiver.
In the example of FIG. 10, since a light interception amount depends on a thickness of the substrate 503, the detection is hardly performed when the substrate 503 is thinned. As shown in FIG. 12, depending on the disposition of the sensor, the light outputted from the phototransmitter 501 is reflected by a surface of the substrate 503 and is incident to the optical receiver 502. Therefore, occasionally the detection of the substrate 503 is obstructed.
When the optical path is set in the oblique direction like the example of FIG. 11, since a light interception state is generated by the surface of the substrate 503, it seems that the light interception amount can be increased to stably perform the detection. However, in the above disposition, the phototransmitter 501 differs from the optical receiver 502 in height, so that it is difficult to align the substrate 503 and the optical path with each other. In order to stably detect the substrate 503 in horizontal behavior, as shown in a region 500 of FIG. 11, it is necessary that a detection region having a width in which the substrate can be included be produced in a certain degree of height range. However, when the optical path is set obliquely, undetectable regions u and v are generated in the region 500 that should be the detection region.
In order to eliminate the undetectable regions u and v, another set of the phototransmitter and the optical receiver is disposed near the phototransmitter 501 and the optical receiver 502 with a positional relationship opposite to that of FIG. 11. However, in such a configuration, wiring becomes complicated to increase cost. Possibly the reflected light generated in the optical path of one of the sensors is incident to the optical receiver of the other sensor to degrade detection accuracy.